A controlled STOP

Fast, powerful and controllable, servomotors are an essential part of many designs. But what happens when power to them shuts down?

Without energy to control motor shaft position, systems tend to move in response to whatever external loads are applied at the time. At best, this means that an emergency shut down or an unexpected loss of power results in a machine out of sync. At worst, uncontrolled motion can present significant safety risk.

Fortunately, spring-applied electromagnetic and permanent magnet or PM brakes can safely bring motors to stop, or hold static position without need for external power.

The most common and cost effective brake for the majority of servomotor applications is the spring-applied design. These operate by applying spring force to a friction plate mounted on the motor shaft. A dc voltage applied to a coil disengages the brake; if the voltage is removed, the brake activates.

Design considerations

When selecting a brake for a servomotor application, think carefully about operating conditions. The brake must have sufficient static (or holding) torque to keep the shaft in position under all operating conditions. The orientation of the motor should be established — especially if its axis is not horizontal — as this can affect loads on the brake. Also:

In a highly dynamic application, the brake hub and friction plate should have minimum inertia; otherwise, this inertia adds to that which must be accelerated and decelerated by the motor, increasing its power requirement.

Brake dimensions must be kept to a minimum for a given static torque. Servomotors usually have a compact overall diameter, and the brake diameter must be smaller, while keeping the brake's overall length to a minimum.

Determine how static brake parts can be fitted to the motor to maintain brake concentricity and squareness to the motor shaft. How is the brake hub to be fastened to the motor shaft, with regard to torque transmission and axial location? Normally a bore and keyway is used; other methods include shrink fit, driving pin, or D connection. If space is restricted, can the hub spline be machined on the motor shaft?

Consider electrical requirements. The power absorbed by the brake coil should be kept to a minimum, as the brake coil is continuously powered while the motor is in use, generating heat in a confined space with high ambient temperatures (up to 150° C in some cases.)

Brake coil wiring into the motor must also be specified, along with the length and termination of any flying leads. Find out if any regulations apply to this wiring and any connectors used.

Brake coils require a dc voltage, normally provided by rectifying the ac voltage used to power the motor. Ensure that the dc voltage and current is always supplied to the brake coil while the motor is running, to keep the brake disengaged — as servomotor speed can be altered by varying supply voltage.

The speed of electrical response in the brake control circuit must ensure that the brake is disengaged before the motor begins rotating. Consider this timing if the motor controls falling loads such as lifts or cranes. Or, for emergency stops (as during loss of power when the motor in a dynamic condition) remember that servomotor brakes have limited ability to absorb energy.

If a brake can handle the energy of a single dynamic stop, the frequency of these stops should be determined to predict the wear life of the friction material.

Backlash

In many safety-critical applications, it is sufficient if a brake can bring its motor to a controlled stop or hold it in approximate position in the event of power loss. However, in some high-precision applications where many different servomotors may operate in exact synchronization, there is a need to control the precise position of the motor shaft. Here, backlash is a concern.

Spring-applied brakes do have small but significant backlash. In many cases, as where a motor is coupled to a gearbox itself exhibiting backlash, brake backlash is lower than that exhibited by other parts of the system and the spring-applied design provides adequate precision.

That said, the need for a zero-backlash brake may call for a different design altogether. Permanent magnet brakes are the choice here: Their magnets generate braking force and dc voltage applied to the coil generates a magnetic force that opposes it; then a release spring disengages the brake.

Using finite element analysis (FEA) tools, design engineers can optimize the magnetic path within the brakes to maximize torque while minimizing power consumption and cost. Also, continuous product development including evaluation and the introduction of special friction materials suitable for high temperature use ensure these brakes are suitable for the most arduous application conditions. Please consult the manufacturer for further information and help with product selection.

For more information, call Matrix Intl., an Altra Industrial Motion Co., in the U.K. at (011) 44 1356 602000 or in the U.S. at (815) 389-3771, or visitmatrix-international.com.

Sidebar: Why this braking?

Why do some servoapplications require the brakes we discuss here? Servomotor applications often require rapid duty cycle with fast rates of acceleration and deceleration. So, servomotors are often made to minimum dimensions — with a compact square section preferable to a short overall length (to keep inertia low) or even a short “pancake” design.

Servomotor brakes are robust and reliable. Even so, their integration requires care and full consideration must be given to the specific characteristics of each motor installation.